Algal and nutrient control system and method for a body of water

ABSTRACT

A system to control nutrients, suspended algae, and filamentous algae in lakes, ponds, and estuaries includes a structure for containing macrophytic vegetation. One embodiment segregates a column of water within a body of water desired for remediation and shields the water column from sunlight sufficiently to kill phytoplankton therein, the lysis thereof releasing nutrients. The released nutrients are sequestered, and remediated water is replaced in the water column with water from the body of water. Floating or submerged macrophytes can be introduced in combination with shading and induced water movement to optimize nutrient uptake by the macrophytes. Water column shading may also be achieved by the macrophytes themselves, or by other means. The macrophytes can include floating aquatic plants, such as water hyacinths, submerged aquatic plants, and terrestrial or aquatic plants caused to float artificially. The invention also deals with toxic algae in a safe and sequestered way.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority from provisional applicationSerial No. 60/227,559, “Macrophyte System and Method for Algae andNutrient Control in a Body of Water,” filed Aug. 23, 2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to systems and methods of waterpurification, and, more particularly, the control of nutrients andsuspended and filamentous algae in estuaries and fresh water bodies.

[0004] 2. Description of Related Art

[0005] Many freshwater lakes and ponds, as well as estuaries, arecharacterized, particularly during the warmer months, by a significantpopulation of suspended algae or phytoplankton in the water body's watercolumn. These largely unicellular plants give the water a greenish andoften a “pea-soup” appearance that many observers find unattractive.Floating mats of unsightly, filamentous algae also can occur. Highconcentrations of algae may lead to low levels of dissolved oxygen inthe early morning hours, leading to stress on the aquatic and fishpopulations. In extreme cases, these conditions will lead to fish killsand the general decline of the quality of a water body.

[0006] The basis of the problem is an abundance of soluble nutrientswithin the water body, which then allows the rapid growth andmaintenance of the elevated population of suspended or filamentousalgae. The source of the soluble nutrients may be sediments, airdeposition, point source polluting discharges, generalized,non-point-source inflows, or most likely a combination of all thesefactors. An effective management strategy would combine elements ofattempting to reduce nutrient loading to the water body with treatmentof the water body itself.

[0007] Currently used methods of controlling algal growth in ponds orlakes typically involve treating the water with selected herbicides or“algicides.” These chemicals kill the suspended algae, returning thewater to its more desired appearance of clear or only slightly coloredwaters. Alternatively, a water body may be treated with various aluminumsalts (e.g., aluminum sulfate), which achieves a similar result througha chemical precipitation reaction. Another strategy employed in managingalgae is to introduce a dye that then, via the mechanism of shading,achieves the same result of killing the algae and returning the water toits algae-free appearance.

[0008] Another problem with these approaches is that the underlyingfeature that initially encouraged the growth of the algae remains; thatis, the nutrients on which the algae grew remain in the water, and afterthe effects of the algicide, herbicide, or dye decrease, the conditionsfor a renewed growth of algae are abundantly present. In addition, forthe algicide and herbicide at least, the negative environmental effectsof potentially toxic accumulation must be considered. In the case ofaluminum salts, a temporary reduction in selected nutrients (e.g.,phosphorus) is effected, but considerable skill and expertise arerequired to effectively and efficiently precipitate the suspended(nutrient-containing) solids.

[0009] An alternative strategy to killing the algae and then creatingrelatively clear but nutrient-rich water body is to cause a release ofnutrients from the algae but then to remove these nutrients from thewater body. A natural method of achieving this nutrient removal isthrough the harvesting of macrophyte vegetation, which takes up thesoluble nutrients as a function of their growth. If the total mass ofnutrients removed through plant harvest were to match the ongoingnutrient loading through the various sources of sediment transport, airdeposition point and nonpoint sources, then the lake or pond would beable to maintain an algal-free appearance.

[0010] In U.S. Pat. No. 4,888,912, a system of growing and harvestingaquatic plants is described. In it, an “envelope,” made of a plasticmaterial permeable to light, air, and water, contains certain plants(typically submerged macrophytes), which then grow and assimilatesoluble nutrients. Because the plants are contained in the envelope,harvesting is convenient and the plants are prevented from escaping intothe larger water body.

[0011] The ability of the plants within the envelope to grow, and hencereduce the population of algae depends on a complex ecologicalcompetition between the introduced species of macrophyte vegetation andthe existing algae in the water body. The nutrients contained in thehealthy algal biomass are normally bound up in cellularconstituents—that is, not biologically available for other plants'growth. The introduced plant species must then effect a decrease in thestanding crop of algae, thereby creating an increase in the amount ofnutrients bioavailable for the introduced species. Therefore, usingintroduced macrophytes to decrease ambient soluble nutrient leve Is andalgae populations is necessarily a two-step process. First, thenutrients bound up in the algal biomass must be released and becomebioavailable. Second, the soluble nutrients must then be incorporatedinto a standing crop amenable to periodic harvesting. The harvestingeffectively removes the nutrients from the water body.

[0012] Shading of the suspended algae-containing water is one means toachieve algal cell lysis and an increase in the proportion of nitrogenand phosphorus in the water column that is biologically available forother plant growth. The use of shading to cause a release of solublenutrients has been investigated and taken advantage of in the prior art.In U.S. Pat. Nos. 5,096,577, 5,180,501, 5,264,127, 5,342,512, and5,409,601 (all assigned to the Lemna Corp.), a floating or submergedaquatic plant is used to cover and shade a wastewater beneath.

[0013] Reddy and DeBusk (1987) determined in a short-term mesocosm-scaleexperiment that the primary nutrient removal mechanism in a waterhyacinth system that received phytoplankton-laden lake water was thesettling of algal cells.

[0014] In U.S. Pat. No. 4,042,367 the introduction of a colored dye isused to prevent the transmission of photosynthetically active radiationthrough a water column. In turn, this action prevents photosynthesis andthereby controls the growth of algae.

[0015] The Florida company Amasek, Inc., working on Lake Apopka andRound Lake in Florida in the late 1980s and early 1990s, grew waterhyacinths within the confines of in situ boom and barrier systems.Through the process of water shading, the hyacinths were able tooutcompete the suspended algae through algal lysis, nutrient release,and subsequent uptake by the hyacinths.

[0016] The sustainable removal of nutrients, however, involves not justshading and subsequent plant uptake and plant harvest. Many macrophytes,such as the floating water hyacinths or submerged macrophytes, arecharacterized by much higher growth rates than can be adequatelysustained by the amount of nutrients held by a water column beneaththem. Reddy et al. (1983) concluded that the high levels of floatingwater hyacinth biomass in a central Florida lake could be sustained onlyby transport of nutrients from the sediments and detritus or by fixation(in the case of nitrogen) from the atmosphere.

[0017] A particular model of a system for decreasing algal concentrationconsists of macrophyte vegetation (plants) and various permutations offloating boom, barrier, and water control mechanisms (pumps, internalbarriers, etc.) for sequentially shading suspended algae-containingwater. The shading causes the algae to lyse and release containednutrients, which then promotes the growth of a standing crop of eithersubsurface (submerged) or floating vegetation. Depending on theconfiguration desired, the plants are periodically harvested from thecontainment system, effectively removing soluble nutrients such asphosphorus and nitrogen from the pond, estuary, or lake. In the case ofsubmerged macrophytes, water chemistry changes caused by theirphotosynthetic activity can further contribute to nutrient removal(e.g., co-precipitation of phosphorus with calcium carbonate) beyondthat achieved by plant harvest. As the overall total level of nutrientswithin the water column decreases, the conditions favorable to sustainednuisance algal growth diminish, and further algal growth is discouraged.

[0018] Therefore, to encourage further plant growth and hence continuedremoval of suspended and filamentous algae, it is believed desirablethat the water beneath the macrophyte vegetation, whether held within abarrier or not, be exchanged with new nutrient-bearing water.

SUMMARY OF THE INVENTION

[0019] It is therefore an object of the present invention to provide asystem and method for water bioremediation.

[0020] It is another object to provide such a system and method thatemploys in situ macrophytes to control nutrient concentration.

[0021] It is an additional object to provide such a system and methodthat avoids the use of toxic additives to the water to be remediated.

[0022] It is a further object to provide such a system and method thatremoves toxic algae without releasing the toxins into the body of water.

[0023] These and other objects are achieved by the system and method ofthe present invention, which is directed to the management and controlof suspended and filamentous algae and excess nutrients in fresh waterbodies and estuaries. A plurality of embodiments are contemplated, abest mode among which is dependent upon the characteristics of eachspecific body of water, as determinable by measurements of waterchemistry and algal decomposition rates.

[0024] One embodiment is a method comprising the steps of segregating acolumn of water within a body of water having excess phytoplankton andnutrients therein and shielding the water column from sunlightsufficiently to kill phytoplankton therein, the lysis thereof releasingnutrients. Next the released nutrients are sequestered, and remediatedwater is replaced in the water column with water from the body of water.

[0025] In some embodiments, floating or submerged macrophytes areintroduced into a body of water in combination with water column shadingand induced water movement to optimize nutrient uptake by themacrophytes. Water column shading may also be achieved by themacrophytes themselves. The macrophytes can include aquatic plants thatfloat on their own, such as water hyacinths, submerged aquatic plants,and terrestrial or aquatic plants caused to float by artificial means.

[0026] Another developing problem in bodies of fresh water is thepresence of toxic algae. At present copper is added to the water to killthe toxic algae, but the difficulty is that this releases the toxinsinto the water.

[0027] The features that characterize the invention, both as toorganization and method of operation, together with further objects andadvantages thereof, will be better understood from the followingdescription used in conjunction with the accompanying drawing. It is tobe expressly understood that the drawing is for the purpose ofillustration and description and is not intended as a definition of thelimits of the invention. These and other objects attained, andadvantages offered, by the present invention will become more fullyapparent as the description that now follows is read in conjunction withthe accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIGS. 1A and 1B are, respectively, elevation and top plan views ofa first embodiment of the system of the present invention.

[0029]FIGS. 2A and 2B are, respectively, elevation and top plan views ofa second embodiment of the system.

[0030]FIGS. 3A and 3B are, respectively, elevation and top plan views ofa third embodiment of the system.

[0031]FIGS. 4A and 4B represent several embodiments of a fourthembodiment of the system, with FIGS. 4A and 4B illustratingcross-sectional and plan views, respectively, of systems having a singlewall enclosure in the body of water, an enclosure having an enclosedbottom, and a partition separating out the water column from a portionof the shore.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] A description of the preferred embodiments of the presentinvention will now be presented with reference to FIGS. 1-4B.

[0033] In a first embodiment (FIGS. 1A and 1B), the system 10 comprisesmeans for shading 11 a first section 91 of a body of water 90. Theshading means may comprise, for example, floating balls, shade cloth, oropaque fabric, etc., although these are not intended as a limitation. Asecond section 92 of the body of water 90 contains macrophytes 12. Themacrophytes 12 may comprise either or both of a floating or submergedvariety. In this embodiment the first section 91 is larger than thesecond section 92.

[0034] In one configuration, the shaded area 91 is equicentric withboomed floating or submerged plants 12, providing for a detention timeby the water under the shading and a subsequent detention time in theproximity of the floating or submerged plants 12. In this way, as thewater moves under the shade 11, it is shaded and subsequently thenutrients released by dying algae are taken up by the macrophytes 12.

[0035] In a second embodiment 10′ (FIGS. 2A and 2B), a floating boom 14containing macrophytes 12 is not circular but instead rectangular,wherein the water enters one end 15, is shaded (causing nutrientrelease) under the shaded area 91, and then moves into the area 92containing the macrophytes 12, and exits the other end 16.

[0036] In a third embodiment 10″ (FIGS. 3A and 3B), a barrier 17 extendsfrom the surface 93 to the bottom 94 and is affixed to a floating boom14 containing the macrophytes 12. Means are provided for controlling theingress and egress of water to this now-separated water column. Thecontrol of the water exchange can be effected by a pump 18, deployedunder the water surface 93 and adjacent the barrier 17. The egress ofthe water is effected by the pressure differential created by theingress pump 18 and is made through an egress vent 21 in another portionof the barrier 17.

[0037] In this embodiment, in which the barrier 17 extends from thefloating boom 14 to the bottom 94 is used, an internal structure orbaffle 19 adapted to cause serpentine flow patterns within the barrierstructure 17 is used. Such a structure has no appreciable effect on thedetention time of the enclosed water (which would be determined by thepumping rate at the ingress structure), but is believed to serve toenhance settling of suspended algae by creating zones of lower watervelocities.

[0038] A fourth embodiment and subembodiments thereof (FIGS. 4A and 4B)is believed at present to represent the best mode of practicing theinvention. In this embodiment, a system and method for reducing apopulation of phytoplankton 20 and excess nutrient level in a body ofwater 32 comprises means for segregating a column of water 31 within thebody of water 32 having excess phytoplankton 20 and nutrients therein.In one subembodiment 30 the segregating means comprises an enclosure 33extending from a surface 93 to a bottom 94 of the body of water 32, as,for example, in FIGS. 4A and 4B. Although this embodiment 30 is shown assubstantially cylindrical, this is not intended as a limitation, andother shapes may be contemplated within the scope of the presentinvention. The enclosure 33 comprises a material impermeable to waterpassage and has floats 34 attached to a top edge 35 of the enclosure 33on the water body surface 93 and further has weights 36 attached to abottom edge 37 for retaining the bottom edge 37 adjacent the water bodybottom 94.

[0039] In a second subembodiment 40, also shown in FIGS. 4A and 4B, thesegregating means comprises a water-impervious enclosure 41 that isinsertable into the water body 32. The enclosure 41 comprises an opentop 42, an enclosed bottom 43, a wall 44 affixed to the bottom 43 andextending upwardly therefrom, and a float 45 attached to a top edge 46of the wall 44. The enclosure 41 therefore creates a water column 47therewithin.

[0040] In a third subembodiment 50, also shown in FIGS. 4A and 4B, thesegregating means comprises a partition that is droppable from thesurface 93 to the bottom 94 of the body of water 32. The partitioncomprises a wall 51 that extends laterally from a first edge 52 that isanchorable against a first location 95 along an edge of the water body32 and a second edge 53 that is anchorable against a second location 96along the water body's edge in spaced relation from the first location95. This partition 51 thereby creates a water column 54 that is enclosedby the partition wall 51 and the water body edge 97 between the first 95and the second 96 location.

[0041] In an alternate embodiment to the third subembodiment 50,particularly useful in a shallow body of water or a shallow portion of abody of water, the partition comprises a row of tall and deeplyextending plants, such as a row of bullrushes or cattails, which willhold the macrophytes in place and substantially prevent rapid waterexchange therethrough.

[0042] The system further comprises a cover for shielding at least aportion of the water column from sunlight, for killing phytoplankton 20therein, which need sunlight to survive. Killing the phytoplankton 20 isbelieved to be a requirement for excess nutrient removal, since thephytoplankton 20 typically outcompete the macrophytes 12 for thenutrients in the water. The lysis of the phytoplankton 20 releasesnutrients therefrom. In the first subembodiment 30, the cover comprisesa plurality of floating or submerged macrophytes 12 placed within thewater column 31. The macrophytes, as described above, have an ability totake up released nutrients from the lysed phytoplankton. Thissubembodiment further comprises means for periodically harvesting themacrophytes 12 to reduce an excess population thereof. Such harvestingmeans are well known in the art, and may comprise commercial harvestingdevices, for example.

[0043] In the second subembodiment 40, the cover comprises asunlight-reducing dye 48 addable into the water column 47.

[0044] In the third subembodiment 50, the cover comprises a mixture 55comprising a sunlight-reducing dye and a liquid immiscible with andlighter than water, the mixture floatable atop the water column 54. Inother embodiments, the cover comprises a shading material 11 such asshown in FIGS. 1A and 1B that is placeable atop at least a portion ofthe water column.

[0045] The system additionally comprises means for sequestering thereleased nutrients, which in the first subembodiment comprises themacrophytes, floating 12 or submerged 12′.

[0046] Finally, the system comprises means for replacing remediatedwater in the water column with water from the body of water. Typicallythe remediated water replacing means comprises a pump for removing waterfrom the water column and for adding water from the water body into thewater column. Preferably, as shown in FIG. 4A for the firstsubembodiment 30, the pump comprises a first pump 60 that is positionedat a first location 62 in the enclosure 33. The first pump 60 is forremoving water from the water column 31 and returning the remediatedwater into the water body 32. A second pump 61 is for adding water intothe water column 31 from the water body 32. The second pump 61 ispositioned at a second location 63 in the enclosure 33 that is loweralong the water column 31 than the first location 62. Further, the firstpump 60 is in laterally spaced relation from the second pump 61.Preferably the first 60 and the second 61 pumps have a pumping rateadapted to effect a steady-state between water remediation and additionof water from the water body 32. One of skill in the art will recognizethat each body of water and its components will have its owncharacteristics, and that the phytoplankton kill rate may vary over asmuch as an order of magnitude from site to site, and also depending onthe amount and quality of shading, and the size and depth of the watercolumn.

[0047] The system in other embodiments may also further comprise acoagulant that can be added to the water column to compact lysedphytoplankton at a bottom of the water column. The coagulant maycomprise, for example, an aluminum compound such as aluminum chloride,which also sequesters phosphorus in the water column.

[0048] The system may also comprise means for removing lysedphytoplankton from the water column, such as a pump for removing aviscous bottom layer from the water column.

[0049] It should be noted that the fourth embodiments of the inventionare ideally suited for removing toxic algae without exposing the rest ofthe body of water to released toxins, since the toxins remain in thewater column and are not released into the water body as a whole. Acopper compound can be used as known in the art, also without exposingthe water body as a whole to the copper.

[0050] In all embodiments means for providing aeration may be included.Aeration may be introduced, for example, via one or more fine diffusers200 (e.g., large air stones) that are deployed at various depths andlocations within the system. The purpose of the aeration is to encouragelateral and vertical mixing (as constrained by the other goal ofallowing algal biomass settling), and under certain circumstances, toencourage (accelerate) decomposition of the settled algal particles.

[0051] In another alternate embodiment an electron acceptor other thanoxygen (such as sulfate) to encourage algal decomposition (and nutrientrelease). Additionally or alternatively, electron donors may be added,such as organic matter, to the bottom of an enclosure to createoxidation-reduction conditions amenable to nutrient release.

[0052] In the embodiment 10″ of FIGS. 3A and 3B, means may also beprovided for exposing the algae in the pumped “ingress” water to somesort of physical or chemical perturbation to accelerate their lysis andnutrient release within the enclosure.

[0053] It may be appreciated by one skilled in the art that additionalembodiments may be contemplated.

[0054] In the foregoing description, certain terms have been used forbrevity, clarity, and understanding, but no unnecessary limitations areto be implied therefrom beyond the requirements of the prior art,because such words are used for description purposes herein and areintended to be broadly construed. Moreover, the embodiments of theapparatus illustrated and described herein are by way of example, andthe scope of the invention is not limited to the exact details ofconstruction.

[0055] Having now described the invention, the construction, theoperation and use of preferred embodiments thereof, and the advantageousnew and useful results obtained thereby, the new and usefulconstructions, and reasonable mechanical equivalents thereof obvious tothose skilled in the art, are set forth in the appended claims.

REFERENCES

[0056] Reddy, K. R.; Rao, P. S. C., DeBusk, W. F. “WaterhyacinthProduction Systems in Nutrient Enriched Waters, Project 2161,” (Sep. 13,1983), GRI/IFAS Project Advisors Review Meeting, University of Florida,Gainesville.

[0057] Amasek, Inc., “Lake Apopka Water Hyacinth Demonstration Project,Semi-Annual Report May-October 1989,” (Jan. 29, 1990), Amasek, Inc.,Cocoa, Fla., prepared for St. Johns River Water Management District,Palatka, Fla.

[0058] Reddy, K. R., DeBusk, T. A., “State-of-the-Art Utilization ofAquatic Plants in Water Pollution Control,” Wat. Sci. Tech. Vol. 19 No.10 pp.61-79, 1987.

What is claimed is:
 1. A method for reducing a population ofphytoplankton and excess nutrient level in water comprising the stepsof: shielding the water from sunlight sufficiently to kill phytoplanktontherein, the lysis thereof releasing nutrients; and sequestering thereleased nutrients.